All combustion control systems include at least an air flow (oxygen) subsystem and a fuel flow subsystem. Many types of control schemes are commonly used by those skilled in the art to control the air/fuel ratio; they are generally characterized as either positional or metering type systems.
Positioning systems are often used in smaller combustion systems and solid fuel units, where one or both flows are not usually measured. The combustion device energy supply controller, whether pressure, flow, and/or temperature based, positions either a single shaft (i.e., commonly called a jack shaft), a fuel flow element, or an air flow element which in turn causes a change in the air and/or fuel flow into the combustion device. The air/fuel ratio is substantially fixed, determined by the mechanical linkage. These systems generally cannot maintain a precise air/fuel ratio when either the air or the fuel characteristics change from the initial ratio calibration. Such systems are generally biased to operate in the inefficient range with very substantial ecess air throughout the load range and normally do not or cannot adjust for daily changes in input air and/or fuel characteristics such as relative humidity, temperature, combustion air supply fan parameters, linkage wear, changes in fuel characteristics, and other problems. There is no correction for unburned carbon losses or loss of combustion volatiles. The combustion control system is adjusted for the expected worst case condition plus an amount of excess air believed to be sufficient to avoid series problems. Such a prior art system is shown in FIG. 2 of the appended drawings.
Metered systems are useful where the air/fuel flows can both be measured. Typically, cross limit controls can be installed in a lead-lag combination such that fuel flow lags air flow when increasing the combustion firing rate, and fuel flow leads air flow when decreasing the combustion firing rate. Such a prior art system is shown in FIG. 3 of the appended drawings.
Optimization of the fuel/air ratio usually involves the use of flue gas analyzers in the exhaust passageway. Various schemes have been employed, some trimming the fuel flow and others trimming the combustion air (oxygen) flow, based on the percent oxygen signal derived from an exhaust gas sensor. The assumption is made with oxygen (and carbon monoxide) analyzer-based controllers that the measurement can be related to the amount of excess combustion air mixing with the fuel in the combustion zone. A control set point indicative of the desired excess air is entered as a controller input. Many problems are associated with such systems. The oxygen (or air) present in the stack may have leaked into the analyzer path upstream of the combustion zone. Many combustion devices, i.e., negative draft and induced draft devices, operate at an absolute pressure which is less than atmospheric. Reducing actual combustion zone air to lower the inferred `excess air` measurement to the set point may result in an actual air deficiency in the combustion zone. This results in the combustion device actually operating at an inefficient level even though the control system indicates optimized operation. From a review of FIG. 4 it can be noted that efficiency drops off more rapidly on the insufficient air side of the efficiency peak than on the excess side. The slope of the efficiency loss from the peak can be 10 to 15 times greater for insufficient air than for the excess air case.
Flue gasses are subject to stratification, thus the gas analyzer must be carefully positioned. An analyzer which is not properly located results in erroneous readings which lead to inefficient operation.
Common oxygen analyzers provide either a percent dry output or a percent wet output. Percent dry analyzers are usually of the sampling type, with the amount of water vapor being condensed. They result in long response times to varying conditions and require high maintenance of the associated analyzer system components (pumps, water cooling, etc.) More modern analyzers are of the zirconium oxide `in situ` type operating according to the well-known Beer's Law. In these units, the probe temperature is above the ignition temperature of the combustibles in the flue gasses. Incomplete reaction products use up available oxygen at the sample point, giving a percent output value which is lower than the actual value, again leading to inefficient operation.
The percent oxygen (or combustion air) set point initially determined as optimum is often not a constant as certain conditions change over time. Such variations include fuel characteristic changes which require more or less air; mechanical efficiency of the burning mechanism can vary, requiring more or less oxygen to avoid forming carbon monoxide or smoke. Since the oxygen controller is always a one-way (increase/decrease) action device (that is, for an increase in measured percent oxygen the controller reduces air to maintain its set point at zero), this action is incorrect on many solid fuels as the combustion chamber is also in fact a fuel drier. When high moisture content fuel is encountered the combustion process slows down and the excess oxygen detected by the stack gas analyzer increases; the subsequent reduction of combustion air by the oxygen controller exacerbates the actual problem and the fuel bed may be extinguished.
Another problem associated with flue gas oxygen analyzers is frequent periodic maintenance and/or accuracy drift. Duplicate equipment for redundancy is expensive. Since the entire control scheme is dependent on the reliability and accuracy of the gas analyzer, and since the analyzer is subjected to a harsh operating environment, failures and out-of-specification drift will cause inefficiencies and system failures. A failure or inaccuracy in the high signal direction (i.e., indicating excess air) can result in an unsafe condition being created as the oxygen controller will decrease combustion air supply. A failure or inaccuracy in the low signal direction can result in high excess air as the controller reacts to the low signal; at low loads this may actually `blow out` the flame by creating a lean fuel mixture.
Other problems encountered with flue gas analyzer systems include high initial installation and continuing maintenance expenses which often cannot be justified. Specifically, fuel savings in smaller combustion devices, or applications where the fuel costs are low, may not offset the costs of an expensive oxygen and/or carbon monoxide analyzer system. Also, many combustion devices (such as metal heating furnaces) operate at temperatures above the upper temperature limit of a conventional oxygen probe and therefore such furnaces lack satisfactory optimization solutions. Many combustion devices do not have room in their combustion zones to install a conventional oxygen and/or carbon monoxide probe properly, and the problem is particularly exacerbating when multiple zone furnaces share a common flue gas outlet, where each combustion chamber must be individually monitored.
Sometimes a carbon monoxide gas analyzer is also installed to overcome some of the foregoing problems. Such an analyzer permits an inference of `peak efficiency` because in theory carbon monoxide is found only as a product of insufficient air in the combustion zone. Unintended air infiltration will only cause a slight dilution in the carbon monoxide measurement.
Current carbon monoxide analyzers require cooling of the necessary electronics to prevent overheating; this requires either air purge blowers or cooling water supplies, which incur failures resulting in analyzer failures. As with the oxygen analyzers, carbon monoxide analyzers require frequent maintenance by highly trained personnel, they are associated with high initial costs, suffer high failure rates, and have relative low maximum temperature limits (e.g., 600 degrees Fahrenheit).
In addition to the multiplied expense of such combination oxygen/carbon monoxide analyzer systems, the carbon monoxide analyzers are subject to `zero point` calibration drift. Conventionally, to recalibrate the analyzer, the excess combustion air is increased, then minimal carbon monoxide inferred in the measurement and the measured value taken as the zero point. However, plugged or cracked burners generate carbon monoxide even at high excess oxygen levels. Thus the inferred zero calibration procedure masks inefficiency and other problems.
In certain applications, and with certain fuels, other serious limitations of oxygen and oxygen/carbon monoxide analyzer systems exist such that they are inefficient or completely inappropriate. For example, on solid or liquid fuels, unburned hydrocarbons are formed prior to carbon monoxide, representing fuel losses which are undetected by the sensors. In superheated steam-producing combustion apparatus, the most economical operating point may not occur at maximum combustion efficiency, since it may be more economical to operate at excess air levels and gain additional superheat temperature.
With solid fuels it is possible to have carbon monoxide form at high excess air levels by physically blowing partially combusted particulate matter off the fuel bed, causing a release of carbon monoxide. Subsequently, the prior art control system will adjust the air/fuel ratio in the wrong direction because it necessarily assumes that carbon monoxide is a product of insufficient air. Unburned carbon losses due to flue gas particulates and unburned flue gas volatiles are not ordinarily considered in determining combustion efficiency. A serious control problem exists in solid fuel grate fired combustion devices, even when equipped with both oxygen and carbon monoxide analyzers. Significant quantities of fuel can be left on the grate and lost into the ash pit even when the oxygen and carbon monoxide systems are properly operating as intended. This loss can be significant and can usually be recovered by adding more combustion air than the sensors indicate is needed. These losses have not generally been considered when determining combustion efficiency. Also, the fuel bed can channel (develop holes) and permit combustion air to pass unreacted through to the analyzers where it is detected and treated as excess air. Here, the efficiency appears higher than it actually is, and unless periodic ash samples are checked for remaining combustibles, the inefficiency will go unnoticed.
U.S. Pat. No. 4,033,712 to Morton attempts to overcome similar limitations by a simple system in which only the exhaust gas temperature (EGT), i.e., the wasted heat, is measured. The Morton patent is directed solely to seeking the air/fuel ratio which produces the maximum combustion produced temperature, as measured by an exhaust temperature sensor which allegedly mesures the EGT. This will not work on an industrial furnace because the exhaust stack gas temperature thereof goes down when excess air is reduced (higher efficiency, see FIG. 4), not as in the Morton patent where the exhaust temperature of the engine goes up. There is no consideration in the Morton patent of the net heat (as opposed to EGT) released in the combustion process, i.e., heat absorbed in the work product, preheaters, auxiliary heaters, heat recovery units, etc. Nor is there any attempt to estimate or calculate the net heat released by the combustion process as an indication of efficiency. In the sole specific use disclosed in the Morton patent, a stationary internal combustion engine's exhaust temperature is maximized.
Also known in the prior art are U.S. Pat. Nos. 3,184,686 to Stanton, and 4,054,408 to Sheffield et al. The controller of the '686 patent closely follows a paper entitled "Optimalizing System for Process Control presented at the 1951 meeting of the Instrument Society of America by Y. T. Li, summarizing the Massachusetts Institute of Technology work of Dr. C. S. Draper. Other related patents include U.S. Pat. Nos. 4,253,404 and 4,235,171 to Leonard; U.S. Pat. No. 4,362,269 to Rastogi; and U.S. Pat. No. 4,362,499 to Nethery.
For the purposes of the present disclosure, the term "blowdown" is considered as the removal of liquids or solids from a process or storage vessel or a line by the use of pressure.